1. Field of the Invention 
The present invention relates to a liquid crystal display device, and particularly, to an in-plane switching liquid crystal display device having circular electrodes that achieves a wide viewing angle in every direction and prevents a color shift.
2. Discussion of the Related Art
Because of the recent developments in various portable electronic devices, research is actively ongoing focusing on several types of flat panel display devices including liquid crystal display (LCD) devices, plasma display panel (PDP) devices, field emission display (FED) devices, and vacuum fluorescent display (VFD) devices. In particular, LCD devices have received much attention because they are light and thin, are simple to mass produce, produce high quality images, and have a low power consumption.
An LCD device includes a liquid crystal panel for displaying images and a driving element for supplying driving signals to the liquid crystal panel. In general, the liquid crystal panel includes first and second substrates and a liquid crystal layer formed by liquid crystal molecules injected between the first and second substrates.
There are various display modes of the LCD devices based on arrangements of liquid crystal molecules which are narrow and long. For example, a twisted nematic (TN) mode is commonly employed in LCD devices due to advantages, such as high contrast ratio, rapid response time and low driving voltage. However, the TN mode LCD device is disadvantageous in that a viewing angle for an observer is undesirably narrow because the liquid crystal molecules are aligned vertically by a vertical electric field. To solve such a narrow viewing angle problem, a new technology, such as an in-plane switching (IPS) mode LCD device, has been proposed. The IPS mode LCD device provides a wider viewing angle by aligning liquid crystal molecules horizontally by an in-plane electric field.
FIG. 1 is a plan view of a pixel region of an IPS LCD device according to the related art, and FIG. 2 is a cross-sectional view along A-A′ of FIG. 1. In FIG. 1, an IPS LCD device includes data lines 100 and gate lines 101 crossing each other and defining pixel regions; a thin film transistor (TFT) T arranged at each intersection of the gate lines 101 and the data lines 100; and pixel electrodes 119 and common electrodes 111 arranged parallel to the data lines 100 in the pixel regions. Each TFT T includes a gate electrode 110 formed on the first substrate 103 and connected to the gate line 101; a gate insulation film 113 formed of a material such as SiNx or SiOx on the gate electrode 110; a semiconductor layer 115 formed on the gate insulation film 113; an ohmic contact layer 116 formed on the semiconductor layer 115; and a source electrode 117 and a drain electrode 118 formed on the ohmic contact layer 116 and respectively connected to the data line 100 and the pixel electrode 119. The common electrode 111 is connected to a common line 105, and the pixel electrode 119 is connected to the drain electrode 118 of the TFT T.
In addition, as shown in FIG. 2, the TFT T and the common electrode 111 within the pixel are formed on the first substrate 103, and the pixel electrode 119 is formed on the gate insulation film 113. A passivation film 120 formed of a material such as SiNx or SiOx is stacked on the TFT T, the pixel electrode 119 and the gate insulation film 113 over the entire surface, and a first alignment film (not shown) is applied thereon, thereby determining a direction that the liquid crystal molecules are aligned. The liquid crystal molecules 102 are aligned along a rubbing direction between the common electrode 111 and the pixel electrode 119 when a voltage is not applied to the electrodes.
Further, the IPS LCD includes a second substrate 104 corresponding to the first substrate 103. In particular, a black matrix 106 for preventing light leakage, a color filter layer 107 including R, G and B color filters, and an overcoat layer 108 are sequentially stacked on the second substrate 104.
Thus, the in-plane switching (IPS) LCD can be viewed from the front side at about 70° in the up/down and leftward/rightward directions. In particular, main viewing angle directions at different two domains formed by zigzag type electrodes are symmetrical, such that color shift is mutually compensated, not causing a color shift. Accordingly, the IPS LCD has a wider viewing angle than a TN mode LCD. However, when comparing with CRT display devices, the IPS LCD still has a narrower viewing angle and more problematic color shifts.